1. Field of the Invention
This invention relates to a method and apparatus for tightening threaded fasteners using a hydraulic torque wrench based on determinations of parameters representative of torque and angle of a threaded fastener.
2. Discussion of the Prior Art
Threaded fasteners (hereinafter referred to as `fasteners`), such as a bolt and nut, a bolt threaded into a bore, or a nut threaded onto a stud or shank, are commonly used to connect two or more members into a solid rigid structure or joint. It is highly desirable that the components of the rigid structure remain in the tightened state at all times, and especially when external loadings such as vibration, shock and static or dynamic forces are applied to them.
To achieve a reliable joint in critical applications, it is important that the correct clamping force be applied by the fastener to the joint. This is to say, the tension in the bolt must achieve a certain value for the joint to be properly clamped. If the bolt tension is too low, it may loosen and cause all clamp force to be removed with attendant damage to the structure. If it is too high, the fastener or clamped parts could fail, also causing damage to the structure.
There are no known methods for measuring bolt tension directly without instrumenting the fastener and/or joint. Instrumenting a joint is expensive and time consuming and therefore seldom done in mass production. Sophisticated inferential methods have therefore been developed to estimate the bolt tension based on known or estimated parameters of the bolted system such as the torque applied to the fastener by the tightening system and/or the angle of advance of the fastener. Such methods include terminating tightening when a certain torque value is reached, a certain angle of advance is reached as measured from a defined point, when the yield point of the joint has been reached and others.
The types of methods used have to some extent been dependent on the types of tools used for tightening the joint. Methods in which tightening was terminated based on both measured torque and angle values have typically required instrumenting the tool to acquire both types of data values. These methods have usually been used with electrically or pneumatically driven tools, where they are practical.
In rugged or very heavy duty applications, where hydraulic torque wrenches are typically used, it is not possible, or at best highly undesirable, to instrument the tools. In such applications, the joint has typically been tightened by terminating tightening in response to reaching a certain torque. This avoids the need to instrument the tool because the torque can be determined from the pressure applied to the wrench. The pressure is a parameter which is representative of the torque applied to the fastener, and can be measured remotely from the wrench, typically at the pump which supplies fluid to the wrench. The pump may include a controller for terminating the flow of fluid to the wrench when the pressure corresponding to the desired torque value is reached.
Another difference between hydraulic and pneumatic or electric wrenches lies in their basic operation. Pneumatic and electric wrenches typically can rotate the fastener during tightening for 360.degree. or much more without stopping, until the desired stopping point is reached. Hydraulic wrenches, on the other hand, are usually operated by a reciprocating hydraulic piston/cylinder device operating through a ratcheting mechanism to turn a socket for the fastener a fixed number of degrees, e.g., 32.degree., each full advance of the piston. Advance of the fastener, and therefore advance of the associated angle and torque, are in stages, with the advance starting and stopping several times in the course of tightening a single fastener, until the final stopping parameter, typically a final pressure, is reached.
Thus, in operation a hydraulic torque wrench socket driver will turn for a certain number of degrees while applying torque to the fastener until it reaches its limit of advance or until the final pressure is reached. If the stroke reaches its limit before the final pressure is reached, the operator of the wrench trips a switch which operates a valve to dump the wrench pressure to tank, allowing the wrench to return to its starting point, by ratcheting around the socket. During the resetting of the wrench, the driven socket of the wrench does not rotate but may recede a small amount due to clearance between the socket and the head of the threaded fastener.
Thus, as the torque wrench tightens the fastener, there is generated a time sequence of torque pulses, each covering a limited angle (e.g., 32.degree.), which causes the fastener to rotate and therefore become tensioned. The space between the torque pulses, when the dump valve is open, is used for resetting the socket driver. The result of this complex operation is that there is a rather severe discontinuous functional relationship between the torque, pressure or other force dependent variables of the system with respect to the angle of advance of the fastener. This exacerbates the problem of applying known fastener tightening methods to the operation of a hydraulic torque wrench.
In the past, the output of hydraulic torque wrenches has been largely controlled by monitoring and regulating the magnitude of applied hydraulic pressure. It is well known in the art of threaded fasteners that because of variations in the coefficients of friction at the threaded engagement and at other sliding surfaces, the tension level (i.e., the clamping force) achieved at a given pressure (torque) level can vary as much as 30%. More sophisticated tightening methodologies are known, such as the "turn-of-the-nut" method disclosed in U.S. Pat. No. 4,106,176, which yield a more accurate clamping force, but require the measurement of angle as well as torque, and have not found practical application in fastener tightening by torque wrenches.